This thesis describes in detail the development of a Theory of Particle Mixtures by which certain aspects of the behaviour of mixtures may be predicted by computer simulation from a knowledge of the mean size, voids ratio and relative density of each of the components. The Theory involves mathematical formulae and geometrical models to explain the roles of void filling and particle interference which determine the properties of mixtures. The practical operation of the theory has been confirmed for application to aggregates, powder pastes, mortars and concretes covering such properties as water demand, cohesion and per cent fines, cement content and plastic density. Derived relationships are examined including those between water demand and cement content The original concept proposed by the author in 1983 has been confirmed in principle and modified in detail on the basis of the current research and extended to include plasticising or water reducing and air entraining admixtures and the properties of air content and compressive strength together with derived relationships such as those between compressive strength and cement content. The influences on concrete strength due to cement strength, aggregate type and age at test are included as well as w/c and air content. Confirmation of the validity of the Theory is provided from an extensive appraisal of the literature and from laboratory tests of aggregates, mortars and concretes. A number of case studies have been included to demonstrate application of the Theory to practice. In particular, a technique has been developed to enable the complete particle size distribution of concrete to be synthesized and optimized by computer simulation for any number of component materials or size fractions; an example is provided for 11 size fractions.